Screw drive mechanisms for converting rotational motion to linear motion are employed in a variety of applications, including cutting, machining, woodworking, and precision manufacturing, such as in silicon chip fabrication. Screw drives may offer improved reliability, speed, and accuracy over other types of drives, such as belt drives.
Conventional screw drive mechanisms, such as the device 10 shown in partial cross-section in FIG. 1, may include a threaded rod 11 with a helical thread 12 set at a given lead or pitch. Positioned on the threaded rod 11 is a nut 14 having a loadbearing channel 16 opposite the helical thread 12 of the threaded rod 11. A plurality of balls 18 are positioned between the load-bearing channel 16 and the helical thread 12. End caps 20 at the ends of the nut 14 contain ball return passages 22 that are aligned with the load-bearing channel 16. The nut 14 may be rotated relative to the threaded rod 11 (or vice versa) to impart linear motion to either the nut or the threaded rod. As the nut 14 and the threaded rod 11 rotate relative to each other, the balls 18 rotate relative to the nut and the threaded rod to reduce friction therebetween.
Conventional screw drive mechanisms may also include a housing 24 positioned radially outward from the nut 14 and coupled to the nut with support bearings 26, allowing the nut 14 to rotate freely within the housing 24. The support bearings 26 may include an inner race 32 adjacent the nut 14, an outer race 34 adjacent the housing 24 and a plurality of balls 19 between the inner race 32 and the outer race 34. Seals 28 are installed at opposite ends of the housing 24 to contain a lubricant between the housing 24 and the nut 14 for lubricating the support bearings 26. The housing 24 may then be attached to a device that is moved linearly relative to the threaded rod 11. For example, the housing may be attached to a table for positioning a machineable workpiece, or to a support arm for positioning a wate jet cutting nozzle.
Conventional screw drive mechanisms may have several disadvantages. For example, after extended use, the balls 19 and the inner and outer races 32 and 34 of the support bearings 26 may wear, causing these components to fit loosely together and resulting in reduced control over the position of the housing 24 relative to the threaded rod 11. Where the housing 24 is coupled to a waterjet cutting nozzle, the inability to accurately control the position of the housing may result in inaccurate cuts. Another disadvantage of conventional screw drive mechanisms is that it may be difficult to service individual components of the mechanism without disassembling the entire mechanism.
One approach to addressing wear in the components of the nut 14 has been to preload the balls 18. For example, U.S. Pat. No. 5,263,381 to Shirai discloses applying a preload force to the balls 18, to eliminate axial gaps between the balls, the helical thread 12 and the load-bearing channel 16. Similarly, U.S. Pat. No. 5,013,164 to Tsukada discloses a preload force for reducing vibration of a sliding member along a rail. However, none of the devices discussed above address the problem of a loose fit between the housing 24 and the nut 14 caused by wear in the support bearings 26. In fact, U.S. Patent No. 5,540,575 to Takuno et al. discloses that preloading face-to-face angular contact bearings may heat the bearings, causing them to seize.